1. Field of the Invention
The present invention relates to a method of fabricating a double pipe which is used in an automotive exhaust system or the like to attenuate vibration.
2. Description of the Prior Art
Automotive exhaust pipes generate noise because of vibration of engines and pulsation of exhaust gas. Numerous efforts have been made to reduce such noise. One attempt is to use a double pipe having a quite small gap between the outer pipe and the inner pipe. The dimension of the gap is set less than the amplitudes of the outer and inner pipes. The outer and inner pipes are mechanically caused to bear against each other, for making their vibrations interfere with each other by making use of the difference in natural frequency between the outer and inner tubes. For this purpose, the gap is required to be as narrow as about 5 to 60 .mu.m. If this gap is too large, then the outer and inner tubes do not bear against each other and so their vibrations do not interfere. At this time, the double pipe vibrates at a combined frequency. If no gap is created, the double pipe vibrates as a single pipe having a thickness equal to the sum of the thickness of the inner pipe and the thickness of the outer pipe. Hence, vibration is not effectively attenuated.
One known method of obtaining such a narrow gap is to use a die and a plug, for accurately enlarging a pipe. Another known method consists in making an adequate difference between the inside diameter of the outer pipe and the outside diameter of the inner pipe by mechanical polishing or other process and then inserting the inner pipe into the outer pipe. A further known method is to use a double pipe having an intermediate layer between the outer and inner pipes. Glass fiber is woven into the intermediate layer to absorb vibration. A still other known method consists in fabricating a double pipe and then filling the gap between the inner and outer pipes with particles of a ceramic, asbestos, or the like.
In the method of machining the inner surface of the outer pipe and the outer surface of the inner pipe separately, even if they can be machined to quite close tolerances, the gap might disappear or become too large, depending on the combination of the inner and outer pipes. For this reason, sophisticated machining techniques are needed. In order to obtain a gap of a desired size, it is necessary to combine an outer pipe and an inner pipe after they are separately measured accurately. This necessitates much labor. Where bending techniques are employed, an external force is usually applied to the outer pipe. The external force is transmitted to the inner pipe via the inner surface of the outer pipe. Therefore, the outer and inner pipes are bent in unlike manner. When the bending is complete and the double pipe is released of the external force, the outer and inner pipes spring back in unlike manner. Then, it is inevitable that the inner pipe is pressed against the inner wall of the outer pipe at every bent portion with a large force. The outer and inner pipes vibrate at the same frequency through the contacting portions. As a result, the damping effect of the double pipe weakens. Further, a double pipe having an intermediate layer for absorbing vibration requires a special manufacturing machine. In the method of filling the gap with a damping material after forming a double pipe, the gap must be made large. Therefore, the outside diameter becomes too large as compared with the inside diameter. In addition, the filling operation necessitates much labor. Furthermore, the gap between the outer and inner pipes is not uniform. In this way, the prior art manufacturing methods including bending techniques for practical usage are unable to offer double pipes capable of reducing noise satisfactorily.